Cooling of electronic components is becoming increasingly significant. The trend in integrated circuit (IC) design, and in particular, central processor units (CPUs), is increased speed and circuit density. This increased speed and density, in turn, causes the IC to generate more heat. Without sufficient cooling, the IC may run slower and suffer degradation leading to a shortened life span.
Compounding this problem is that circuit boards are typically housed in enclosures that are increasingly becoming smaller in size. For example, 1U enclosures allow for a circuit board height of less than 1.75″. Additionally, circuit boards stacked in a rack chassis are typically tightly spaced together, further complicating cooling designs.
A system fan is typically utilized to cool the components in an electronic housing. FIG. 1 shows a typical computer enclosure 1. One or more circuit boards 3 containing ICs are located inside a chassis housing 2 in various orientations. A system fan 4 is mounted on one side of chassis housing 2, typically the rear of chassis 2 for aesthetic purposes. System fan 4 creates airflow that removes heat generated by the ICs.
Various concerns arise when implementing system fan 4. The airflow generated in chassis 2 must direct air across chassis 2 so that components located opposite fan 4 will be cooled. Such airflow will be uneven due to the various flow impedances created by the components in chassis 2. Additionally, airflow reaching those components located furthest from fan 4 will have reduced velocity, which in turn decreases the amount of heat that can be removed. Furthermore, system fan 4 is also a single point failure with regards to cooling, making reliability of fan 4 significant. Loss of fan 4 may be catastrophic and bring down the entire system. While additional fans may be utilized, this is typically not an option due to decreasing size requirements.
Heat sinks with integrated axial fan(s) are also used. The fan sits atop the heat sink, which is typically mounted to the top surface of an IC. Heat dissipated from the IC is transferred to the heat sink, where it is removed by airflow generated by the fan. This approach has several drawbacks. The thermal efficiency of a heat sink fan is hard pressed to cool the latest high powered CPUs. Attempts to increase the thermal efficiency of the fan by increasing the fan's propeller's rotational speed forces the fan to consume more power, which stresses the fan's motor bearings and typically leads to degradation in fan reliability. Hot air reflected back from the heat sink also adversely affects the fan's bearings and reliability. Additionally, only the IC below the fan is cooled. Other components on the board must be cooled using alternative methods, such as a system fan. Furthermore, mounting the fan atop the heat sink stresses already tight size requirements.
Instead of a fan, blowers that sit atop the heat sink have also been used, with the airflow generated by the blower directed down and through the heat sink by use of a manifold. The main difference between fans and blowers is in their flow and pressure characteristics. Fans deliver air in an overall direction that is parallel to the fan blade axis and can be designed to deliver a high flow rate, but tend to work against low pressure. In comparison, a blower 21 delivers air in a direction that is perpendicular to the blower's impeller 22 axis, typically at a relatively low flow rate but against high pressure, as shown in FIG. 2. Blowers can produce approximately three times more static pressure than a fan, making blowers more suitable for cooling high-powered CPUs. However, as with fans, mounting a blower atop a heat sink increases the height profile of the circuit board.
Blowers have also been mounted in the chassis away from the electronic components. The blower may act as a system fan, or alternatively airflow is directed to various boards or electronic components using air ducts. This adds complexity to the packaging and manufacturing process. Additionally, the blower or blowers, located apart from the circuit board, take up much needed space elsewhere in the enclosure.
To cool very high-powered components on circuit boards, liquid cooling driven by a pump that delivers either chilled or room temperature water has been used. Central pumps within the enclosure or alternately, an external pump, are typically used. Historically, the relatively large size of the pump has been impractical for today's increasingly smaller sized enclosures and parallel oriented circuit board rack assemblies. In addition to consuming space within the enclosure, using an external or internal system pump requires interconnects to each circuit board cooled which may be a source of leakage over time. Additionally, a centralized fan, or fans, is typically used to move air within the system, which, as described above, has several drawbacks and is inherently not sufficient to cool each of the various components in the system.
Alternatively, refrigerant-cooled systems have been used. Again, the large size of the compressor and the use of fans have historically made use in smaller sized enclosures or in parallel circuit board rack assemblies impractical.
Adding further complications is that the thermal characteristics of electronic components on a given circuit board are difficult to forecast and typically cannot be determined without actual experimentation. This experimentation often entails switching between various types of cooling methodologies, such as air, liquid, and refrigerant cooled systems, each system requiring additional effort and cost to implement. Additionally, upgrading circuit boards with, for example, the latest high-powered processor, often require costly modifications to upgrade thermal cooling capacity.